In order to activate and diagnose the airbag arranged in the steering wheel of a vehicle (driver's airbag), an arrangement is required which makes possible unimpeded signal transfer between the activation system of the airbag in the steering wheel and a control unit arranged outside the steering wheel, regardless of steering wheel position. This signal transfer was usually accomplished by means of slip rings or volute springs. As is already evident from German Patent No. 38 12 633 A1, mechanical contact devices of this kind interfere with steering wheel movement. In addition, they do not always guarantee unimpeded operation; in particular, because of relatively high resistance fluctuations, they falsify diagnosis of the activation system. The resistor of a firing capsule for the airbag usually has a very low resistance of about 2 ohms. For diagnosis of the airbag, this resistance is repeatedly measured and a fault message is reported to the driver via a warning light if the measured resistance exceeds or falls below the actual resistance of the firing capsule by a defined threshold. To allow a reliable diagnosis to be performed, a measurement of the resistance of the firing capsule must be guaranteed with very high accuracy; in other words, the system for signal transfer from the airbag to the control unit must exhibit only very small resistance fluctuations as measured at the firing capsule resistor. According to German Patent No. 28 12 633 A1 and German Patent No. 24 33 555, there is inserted between the steering wheel and steering column, instead of slip rings or volute springs, a transfer unit whose primary and secondary windings are mounted rotatably with respect to one another about the steering wheel axis. German Patent No. 24 33 555 discloses a transfer unit whose primary and secondary windings are located in separated dished cores which are arranged coaxially rotatably with respect to each other. This known transfer unit allows a relatively large leakage in the magnetic flux between the primary and secondary windings, so that the coupling between the two windings is not very close, and the accuracy with which the firing capsule resistance is measured suffers thereby.
A transfer unit of the type cited initially, in which the primary and secondary windings are each located in a U-shaped dished electrode, is known from European Patent No. 0 520 535 A. The two U-shaped dished cores are located one above another in the direction of the rotation axis, the end faces of the arms of U-shaped dished cores lying opposite one another. As a result, the magnetic flux through the two dished cores intersects the air gaps existing between the end faces of the arms in the direction of the rotation axis. A mutual axial offset of the two dished cores would modify the air gap, which has a very pronounced effect on magnetic coupling between the two windings of the transfer unit and correspondingly influences the accuracy with which the firing capsule resistance is measured.
It is therefore the object of the present invention to indicate a transfer unit of the type cited initially which makes possible a measurement of the resistance of the firing capsule with the greatest possible accuracy.